1. Field of the Invention
The present invention relates to an active infrared sensor which may be used in security systems and the like. In particular, the present invention pertains to an improvement for making variable the transmitted optical output from an optical transmitter means.
2. Description of the Related Art
As disclosed, for example, in Japanese Patent Application Publication Kokai No. H13-188970 (2001), applications in which active infrared sensors are used in security systems to detect the entry of persons into protected areas are conventionally known. Such sensors are typically equipped with optical transmitters employing internal optical transmitter elements, and optical receivers employing internal optical receiver elements. Such optical transmitter(s) and optical receiver(s) might be arranged in opposing fashion so as to straddle a protected area such that infrared beam(s) from the optical transmitter(s) is/are transmitted toward the optical receiver(s). Moreover, when infrared beam(s) being transmitted from the optical transmitter(s) to the optical receiver(s) is/are interrupted by intruder(s), causing a change in the amount of light that is received by the optical receiver element(s), a security camera might, for example, be activated or a security company might be contacted.
However, changes in environment and/or conditions under which they are used may cause the optical receiver elements of such infrared sensors to become saturated, thereby preventing satisfactory detection. A specific description thereof follows. Various types of such infrared sensors are available, corresponding to the different sizes of protected areas in which they are intended to be used. For example, there are sensors for use with distances of on the order of 100 m between the optical transmitter and the optical receiver, there are sensors for use with distances of on the order of 20 m therebetween, and so forth. A transmitted optical output from the optical transmitter is set in advance so as to be higher in the case of the former as compared with the latter.
Moreover, when the former—i.e., infrared sensors intended for separations of 100 m—are used in applications involving comparatively narrow protected areas, e.g., where the distance between optical transmitter and optical receiver is on the order of 20 m, the intensity of the so-called feedback beam which is produced when infrared light that is reflected by objects (e.g., wall surfaces or ground surfaces) in the vicinity of the sensor other than the objects being detected and which irradiates the optical receiver can become comparatively large. As a result, despite the fact that an intruder or the like may have passed between an optical transmitter and an optical receiver, interrupting the infrared beam therebetween, because this feedback beam irradiates the optical receiver, the optical receiver is unable to detect an interruption of the infrared beam by the intruder or the like, which results in an undetected intrusion event. Particularly where water has collected on the ground as a result of rainfall or snow has accumulated as a result of snowfall, there is a tendency for the intensity of this feedback beam to become large, thereby increasing the likelihood of occurrence of an undetected intrusion event. Furthermore, during times of such rainfall or snowfall, it is possible that the intensity of the feedback beam will increase and that an undetected intrusion event will occur even where the infrared sensor which is employed is of a type that is designed for the size of the protected area in question (e.g., where optical transmitter(s) and optical receiver(s) of infrared sensors which are intended for separations of 20 m are installed such that they are separated by on the order of 20 m).
In order to remedy such shortcomings, it has been proposed, as disclosed in Japanese Patent Application Publication Kokai No. H5-174260 (1993), for example, that the transmitted optical output from the optical transmitter(s) be made variable. That is, a constitution is adopted wherein the optical transmitting circuit is equipped with a current limiting circuit, and the resistance of a variable resistor which is provided at this current limiting circuit is varied as necessary so as to permit an adjustment of the transmitted optical output. For example, in the event of the aforementioned circumstances tending to cause the intensity of the feedback beam to become large, resistance at the variable resistor might be increased so as to reduce the transmitted optical output. This allows for the intensity of the feedback beam to be held to a low value, thereby permitting an accurate detection of the interruption of the infrared beam as a result of passage therethrough by the foregoing intruder or the like.
However, because the means for making transmitted optical output variable which is disclosed in the foregoing publication requires complicated electrical circuitry, in practice, it is only actually possible to switch between on the order of two levels of transmitted optical output.
Moreover, with a device such as this, which only permits switching between on the order of two levels, depending on the environment and/or the conditions under which the infrared sensor is used, it may not be possible to completely eliminate the aforementioned shortcomings which are caused by the feedback beam.
While a constitution that would permit switching among multiple levels of transmitted optical output has therefore been desired, a practical solution has been difficult because of the concomitant increased complexity in electrical circuitry which would result therefrom as described above.
The present invention was conceived in light of such issues, and therefore, an object of the present invention is to make it possible for the transmitted optical output in an active infrared sensor to be made variable without the need for complicated electrical circuitry, and to make it possible to carry out multilevel adjustment of transmitted optical output as a result thereof.